When optically designing a corrective ophthalmic lens, attempts are made nowadays to take better account of individual geometrico-morphological parameters associated with the wearer and with the selected frame, with this being referred to as “personalized” optical design. These parameters comprise in particular the three-dimensional configuration of the lens relative to the wearer's head, under wearing conditions. This three-dimensional configuration is determined: i) by the orientation of the lens relative to the corresponding eye of the wearer; and ii) by the distance between the lens and the corresponding eye of the wearer.
In order to determine this three-dimensional configuration, the optician places a pair of presentation lenses on the wearer's nose. The presentation lenses comprise the frame selected by the wearer, together with non-corrective lenses that are mounted in the rims of the frame.
The distance between each lens and the corresponding eye of the wearer is measured manually: the optician observes the wearer in profile and estimates a measurement of the distance between the cornea and the rear face of the presentation lens, with the help of a small ruler.
Proposals have been made to automate this measurement by photographing the wearer in profile and processing the image acquired in that way in an attempt to determine the distance between the lens and the eye. Nevertheless, the operation of processing the image has been found to be unreliable, insofar as the rims and the temples of the frame can mask the eye, and insofar as recognizing the point on the rear face of the lens that faces the eye is found to be risky. Furthermore, taking a profile image of the wearer is in addition to taking other geometrico-morphological measurements of the wearer, such as measuring the pupillary distance and the heights of the eyes, the curvature of the frame, the pantoscopic angle of each lens when worn (i.e. the angle formed between the general plane of the lens and the vertical), or indeed the visual behavior of the wearer (in particular the wearer's tendency to move the eyes or the head to a greater or a lesser extent when scrutinizing a field of view). Unfortunately, all or some of those other measurements can be performed by taking one or more images of the wearer in face view. The Applicant has therefore set out, in the context of the present invention, to propose a method that enables the distance between the eye and the lens to be measured on the basis of images that are generally face views and not profile views.
Furthermore, the distance between the pupil and the rear face of the lens is not the magnitude that is the most pertinent when calculating a personalized optical design by ray tracing for the corrective lens that is to be fitted to the frame. Movement of the eye can generally be considered as being a combination of rotations about a particular point that is referred to as the center of rotation of the eye (CRO). It is the position of this point that the designer of the lens desires to know in order to perform calculations properly. In present practice, the position of the CRO is deduced approximately from the position of the cornea by assuming a mean value for the radius of the eye, typically a value of about 15 millimeters (mm). Unfortunately, the radius of the eye varies significantly from one individual to another, such that this approximation leads to significant errors that are highly penalizing for the pertinence of the personalized optical design calculation.